Printed wiring board and a method of production thereof

ABSTRACT

A printed wiring board has an insulating resin substrate having a first surface and a second surface, the insulating resin substrate having one or more penetrating-holes passing through the insulating resin substrate from the first surface to the second surface, a first conductor formed on the first surface of the insulating resin substrate, a second conductor formed on the second surface of the insulating resin substrate, and a through-hole conductor structure formed in the penetrating-hole of the insulating resin substrate and electrically connecting the first conductor and the second conductor. The penetrating-hole has a first portion having an opening on the first surface and a second portion having an opening on the second surface. The first portion and the second portion are connected such that the first portion and the second portion are set off from each other.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of PCT/JP2007/053455 whichclaims the benefit of priority to Japanese Patent Application No.2006-044968, filed Feb. 22, 2006. The contents of those applications areincorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is related to a printed wiring board in which aplated conductor or plated conductors are formed in penetrating-holesformed in an insulating material, and specifically to a printed wiringboard having a plated conductor with improved conduction and a substratewith improved strength, and a method of producing such a printed wiringboard.

2. Discussion of the Background

In a conventional printed wiring board, filled-type through-holes areformed by plating a metal in penetrating-holes formed in an insulatingresin substrate. These filled-type through-holes are formed as platedthrough-holes for electrically connecting conductor circuits formed onthe front and back surfaces of the insulating resin substrate in theprinted wiring board.

For example, Japanese Unexamined Patent Publication No. 2004-311919describes a method of forming through-holes. To form a through-hole, apenetrating-hole 2 is formed in an insulating resin substrate 1 as shownin FIG. 9 (a), and a seed layer 3 made of metal is formed by electrolessplating on a surface of the insulating resin substrate 1, including theinner wall of the penetrating-hole 2 as shown in FIG. 9 (b). Then,electrolytic plating is performed to form an electrolytic plated layer 4on the seed layer 3 as shown in FIG. 9 (c), and by further performingthe electrolytic plating process, a through-hole 6 is formed by fillingthe penetrating-hole 2 with metal as shown in FIG. 9 (d). In athrough-hole formed in such a method as shown in FIG. 9 (d), a void 8 isprone to be formed in its inside.

According to another method as shown in FIGS. 10 (a) to (b), apenetrating-hole 2 having a shape symmetrically tapering toward themidpoint of an insulating resin substrate 1 as shown in a verticalcross-section of FIG. 10( a) is formed in an insulating resin substrate1, and a through-hole 6 is formed by filling metal into thepenetrating-hole 2, i.e., by performing electroless plating to form aseed layer made of metal and an electrolytic plating process over theseed layer as shown in FIG. 10 (a) to (b).

The contents of the foregoing publication is incorporated herein byreference in their entirety.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a printed wiring boardhas an insulating resin substrate having a first surface and a secondsurface, the insulating resin substrate having one or morepenetrating-holes passing through the insulating resin substrate fromthe first surface to the second surface, a first conductor formed on thefirst surface of the insulating resin substrate, a second conductorformed on the second surface of the insulating resin substrate, and athrough-hole conductor structure formed in the penetrating-hole of theinsulating resin substrate and electrically connecting the firstconductor and the second conductor. The penetrating-hole has a firstportion having an opening on the first surface and a second portionhaving an opening on the second surface, and the first portion and thesecond portion are connected such that the first portion and the secondportion are set off from each other.

According to another aspect of the present invention, a printed wiringboard has a core substrate, a resin insulating layer and an externalconductor circuit. The core substrate includes an insulating resinsubstrate having a first surface and a second surface. The insulatingresin substrate has one or more penetrating-holes passing through theinsulating resin substrate from the first surface to the second surface.The core substrate further includes a first inner conductor formed onthe first surface of the insulating resin substrate, a second innerconductor formed on the second surface of the insulating resin substrateand a through-hole conductor structure formed in the penetrating-hole ofthe insulating resin substrate and electrically connecting the firstconductor and the second conductor. The resin insulating layer is formedover one of the first surface and second surface of the insulating resinsubstrate of the core substrate. The external conductor circuit isformed over the resin insulating layer and the core substrate. Thepenetrating-hole has a first portion having an opening on the firstsurface and a second portion having an opening on the second surface,and the first portion and the second portion are connected such that thefirst portion and the second portion are set off from each other.

According to yet another aspect of the present invention, amanufacturing method for producing a printed wiring board includesproving an insulating resin substrate having a first surface and asecond surface, forming a first portion of a penetrating-hole having anopening on the first surface of the insulating resin substrate, forminga second portion of the penetrating-hole having an opening on the secondsurface of the insulating resin substrate such that the second portionis connected to the first portion and that the first portion and thesecond portion are set off from each other, forming a first conductor onthe first surface of the insulating resin substrate, forming a secondconductor on the second surface of the insulating resin substrate, andforming a through-hole conductor structure formed in thepenetrating-hole of the insulating resin substrate to electricallyconnecting the first conductor and the second conductor.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a cross-section view showing the shape of a through-hole in aprinted wiring board according to an embodiment of the presentinvention;

FIG. 2 is a schematic view showing the cross-section of a neck portionof the through-hole shown in FIG. 1;

FIG. 3 is a cross-section view showing an example when the position ofthe center of gravity varies for adjacent through-holes according to anembodiment of the preset invention;

FIGS. 4 (a) to (d) are cross-section views showing steps for producing aprinted wiring board according to an embodiment of the presentinvention;

FIGS. 5 (a) to (d) are cross-section views showing steps for producing aprinted wiring board according to an embodiment of the presentinvention;

FIGS. 6 (a) to (d) are cross-section views showing steps for producing aprinted wiring board according to an embodiment of the presentinvention;

FIGS. 7 (a) to (b) are cross-section views showing deformation where thecenters of gravity of a through-hole is considerable;

FIG. 8 is a cross-section view showing a through-hole made by fillingfiller into a space enclosed by through-hole conductor formed on theinner wall of a penetrating-hole according to an embodiment of thepresent invention;

FIGS. 9 (a) to (d) are cross-section views showing the productionprocess of a printed wiring board according to a background art; and

FIGS. 10 (a) to (b) are cross-section views showing through-hole shapeson other printed wiring boards according to a background art.

DESCRIPTION OF THE EMBODIMENTS

The embodiments will now be described with reference to the accompanyingdrawings, wherein like reference numerals designate corresponding oridentical elements throughout the various drawings.

According to an embodiment of the present invention, a printed wiringboard has a through-hole (also referred to as a “through-hole conductor”or a “through-hole conductor structure”) which is formed in aninsulating resin substrate by forming a plated conductor inside apenetrating-hole formed in the insulating resin substrate, thethrough-hole has a first opening on the front surface of the insulatingresin substrate and a second opening on the back surface of theinsulating resin substrate, and the first and second openings have theaxes of their centers of gravity which are out of alignment or off setwith each other. The “axis of the center of gravity” is defined hereinas a straight line passing through the center of gravity of an openingportion of a through-hole, projecting on the front surface (or backsurface) of the substrate or core substrate and making substantivelyperpendicular to the front surface (or back surface) of the substrate orcore substrate (for example, in FIG. 1, a straight line passing a firstor a second opening on the first or the second surface of the substrateor core substrate and making substantively perpendicular to the firstsurface or the second surface of the substrate or core substrate). Sucha printed wiring board may be used as a core substrate for a multilayerprinted wiring board with multiple conductor layers and resin insulatinglayers alternatively formed on the core substrate.

By setting the axes of the centers of gravity of the opening portions ofa penetrating-hole projecting on the front and back surfaces of aninsulating resin substrate out of alignment or off set, the area of theneck portion of the through-hole (i.e., the cross-section area that iscut by a plane parallel to the front or back surface of the insulatingresin substrate is minimized) becomes out of alignment; therefore, evenwhen the insulating resin substrate is warped, the area on which stressis applied becomes wider for the amount of misalignment of the neckportion. Also, because the front surface of the core substrate andplanes made by connecting the centers of gravity of neck portions ofthrough-holes do not come in alignment with each other, the stress isreduced, making it difficult for cracks to form around the neckportions. Consequently, poor connection due to cracking becomes lesslikely to occur, and mechanical strength of the substrate is improved.

Alternatively, the through-holes may be formed so that the centers ofgravity of the neck portions differ among adjacent through-holes.

In addition, because the cross-sectional area becomes larger around theneck portion of the through-hole, the conduction resistance decreasesand the electric property of the substrate improves.

In addition, in a multilayer printed wiring board according to anembodiment of the present invention, it is preferable to form in a partof the outermost conductor layer pads for connecting bumps at apredetermined pitch and to keep out-of-alignment or misalignment of thecenters of gravity for through-holes projected on the front and backsurfaces of the insulating resin substrate as well as to form the pitchbetween adjacent through-holes similar to the pitch of the pads forconnecting bumps.

As an insulating resin substrate used in an embodiment according to thepresent invention, it is preferable to use a hard substrate selectedfrom a glass cloth epoxy resin substrate, glass clothbismaleimide-triazine resin substrate, glass cloth polyphenylenetherresin substrate, non-woven aramid-epoxy resin substrate, and non-wovenaramid-polyimide resin material, more preferably a glass cloth epoxyresin substrate.

The thickness of an insulating resin substrate is preferably about100-500 μm. If the thickness is less than 100 μm, the rigidity becomesinsufficient, and if it is thicker than 500 μm, plating into athrough-hole or through-holes becomes difficult, which may result invoids in plating.

Conductor circuits formed on both surfaces of an insulating resinsubstrate is, as described later, preferably formed after platingthrough-holes and by etching on a metal foil laminated on both sides ofthe insulating resin substrate and the plated layer formed thereon.

For a substrate having an insulating resin substrate and a metal foil, adouble-sided copper clad laminate obtained by laminating andheat-pressing prepreg and cooper foils. The prepreg may be a B stageprepreg obtained by impregnating glass cloth with epoxy resin. Such asubstrate has good positioning accuracy, because there is nodisplacement of a wiring pattern or via positions during handling aftercopper foils are etched.

Referring to FIG. 1, a through-hole according to an embodiment of thepresent invention has a first opening portion having a shape in whichthe diameter decreases inward from one side of an insulating substrateand a second opening portion having a shape in which the diameterdecreases inward from the other side of the insulating substrate, andthe second opening portion is connected to the first opening portionaround the mid point between the surfaces of the insulating resinmaterial. A neck portion is formed at the point at which the firstopening portion and the second opening portion coincide. In other words,a penetrating-hole having a shape of two connecting truncated cones isfilled with plating and is out of alignment along the substrate surfacedirection by a predetermined distance. The axes of the centers ofgravity (straight lines passing through the centers of gravity of thefirst opening portion and the second portion and making substantivelyperpendicular to the front (and back) surface of the substrate) of thefirst opening portion and second opening portion each projecting throughone side and the other side of the insulating resin substrate are out ofalignment with each other.

It is preferable to form the through-hole described above by laserprocessing a penetrating-hole having a first opening portion and asecond portion, and then by filling the penetrating-hole with metalplating.

Furthermore, to improve the degree of efficiency of absorbing the laserbeam during the laser processing, it is preferable to perform a blackoxide process on the metal foil on the insulating resin substrate inadvance.

To form a penetrating-hole for forming a through-holes with laserprocessing, first, a laser is irradiated from a predefined positiontoward one side of an insulating resin substrate to form a first openingportion in which the diameter decreases inward from the one side of theinsulating resin substrate and that extends around the center from theone side of the insulating resin substrate. Then, to a position out ofalignment from the predefined position and on the other side of theinsulating resin substrate across from the predefined position, a laseris irradiated on the other surface of the insulating resin substrate. Itis preferable to form a penetrating-hole for forming a through-hole byconnecting the first opening portion and the second opening portionaround the center of the insulating resin substrate, and the secondopening portion is formed whereby its diameter decreases inward from theother surface of the insulating resin substrate and extends around thecenter from the other surface of the insulating resin substrate.

To form a penetrating-hole for the formation of a through-hole by usinglaser in an insulating resin substrate, there are, for example, twomethods: the direct laser method for piercing both a metal foil and aninsulating resin substrate at the same time with laser irradiation, andthe conformal method for penetrating an insulating resin substrate withlaser irradiation after removing a part of a metal foil by etching.

It is preferable to perform the laser process described above with apulse-oscillating type carbonic acid gas laser device. The processconditions may be determined by a shape of a penetrating-hole (the firstopening portion and the second opening portion) for forming athrough-hole in which the diameter decreases inward from the surface ofthe insulating resin substrate, i.e., the angle defined by the surfaceof the insulating resin substrate and the side wall of apenetrating-hole (hereinafter referred to as a “taper angle”), and thedepth of the penetrating-hole.

For example, by using on the pulse interval in a range of 10-20 μs andthe number of shots in a range of 1-5, the taper angle and the depth ofa penetrating-hole for forming a through-hole may be adjusted.

Furthermore, a penetrating-hole for forming a through-hole formed underthe process condition described above, the diameter (shown by thereference character “X” in FIG. 1) of the neck portion inside theinsulating resin substrate is preferably 50-250 μm. If the diameter isless than 50 μm, it is too narrow, so the connection reliability of athrough-hole filled with plating is poor. If the diameter is more than250 μm, it is prone to cause voids in a through-hole filled withplating. Therefore, within the range described above, it is less likelythat a through-hole has voids, providing superior connectionreliability.

Moreover, for the misalignment of the centers of gravity for apenetrating-hole for forming a through-hole described above ispreferably within a range of 5-30 μm. If the misalignment is less than 5μm, the stress reduction effect is small. If the misalignment is morethan 30 μm, the shape of a penetrating-hole becomes prone to deformationas shown in FIGS. 7 (a) to (b).

In addition, the pitch between adjacent through-holes is preferably10-400 μm. If the pitch is less than 100 μm, the insulation reliabilityis low. If the pitch is more than 400 μm, it is not suitable for fining.

Still further, it is preferable to perform the desmear process to removeresin residue left on the side of a penetrating-hole formed by laserirradiation. This desmear process may be performed by a wet process suchas a chemical reaction of acids or oxidants (e.g., chromic acid,hydrogen permanganate), by a dry process such as oxygen plasma dischargetreatment, corona discharge treatment, ultra violet laser processing, orby an excimer laser process.

A particular method among these desmear process methods may be selecteddepending on the type or thickness of an insulating substrate, theopening diameter of a via-hole, or the laser irradiation conditions,considering the smear amount expected to be left.

In an embodiment according to the present invention, a through-hole maybe formed by filling with plating in a penetrating-hole. For example, anelectroless plated film may be formed by normal nonelectrolytic platingon the inner wall of the penetrating-hole, and then the through-hole maybe filled with plating via an electrolytic plating process such as thesparger plating method, which causes the plating solution to flow ontothe substrate.

For the nonelectrolytic plating or electrolytic plating described, it ispreferable to have metal plating such as copper, tin, silver, varioustypes of solder, copper/tin and copper/silver, and, nonelectrolyticcopper plating or electrolytic copper plating is preferable.

In an embodiment according to present invention, a conductor circuit maybe formed on both sides of an insulating resin substrate via an etchingprocess of a conductor layer formed at the time when the platedthrough-holes is formed.

In this conductor circuit formation process, first, photosensitive dryfilm resist is laminated onto the surface of the conductor layer, andthen exposed according to a predefined circuit pattern and developed toform an etching resist, and by etching a part or parts of the conductorlayer where etching resists are not formed, a conductor circuit patternincluding electrode pads may be formed.

In the process described above, for an etching solution, at least onetype of aqueous solution may be used among sulfuric acid hydrogenperoxide, peroxosulfates, copper chloride, or iron chloride.

Furthermore, as a pre-process for forming a conductor circuit by etchingthe conductor layer described above, to make fine pattern formationeasier, the thickness may be 1-10 μm, or more preferably about 2-8 μm,by etching the entire surface of the conductor layer in advance.

With such a printed wiring board as a core substrate, a multi-layerprinted wiring board may be formed by forming a build-up wiring layer byalternately forming a conductor layer and a resin insulating layer onthe core substrate according to a normal method.

In such a multi-layer printed wiring board, it is preferable to make apart or parts of the outermost conductor layer pads for connecting bumpswith a predetermined pitch similar to the pitch between adjacentthrough-holes filled with plating in the core substrate. With thisstructure, the wiring resistance via chips to be mounted on PKG may bereduced so that it has the advantage of ensuring a power supply.

Below is a description of an exemplary method for producing a printedwiring board according to an embodiment of the present invention.

(1) To produce a printed wiring board according to an embodiment of thepresent invention, an insulating resin substrate laminated with copperfoils on both sides may be used as a starting material.

For an insulating resin substrate, a hard laminated substrate may beselected from glass cloth epoxy resin substrate, glass clothbismaleimide-triazine resin substrate, glass cloth polyphenylenetherresin substrate, non-woven aramid-epoxy resin substrate, and non-wovenaramid-polyimide resin material, preferably a glass cloth epoxy resinsubstrate.

The thickness of the insulating resin substrate is preferably in therange of about 100-500 μm. If the thickness is less than 100 μm,rigidity is insufficient. If the thickness is more than 500 μm, it isdifficult to fill plating in a penetrating-hole, which may cause voids.

To form a penetrating-hole for forming a through-hole by using a laserin the insulating resin substrate, there may be two methods: the directlaser method for penetrating both the copper foil and the insulatingresin substrate at the same time by laser irradiation, and the conformalmethod for penetrating the insulating resin substrate with laserirradiation after a part of the metal foil for a penetrating-hole isremoved by etching. The thickness of this copper sheet may be adjustedby half-etching.

A double-sided copper clad laminate may be used for the insulating resinsubstrate with copper foils. Such a laminate may be obtained byimpregnating a glass cloth with epoxy resin and laminating and heatpressing a copper foil.

This is because such a laminate provides good position accuracy, asthere is no displacement of the wiring pattern or via position duringthe process after the copper sheets are etched.

(2) Next, a penetrating-hole for forming a through-hole is made in theinsulating resin substrate by laser processing.

When using a double-sided copper clad laminate to form a circuitsubstrate, first, laser irradiation is performed from a predefinedposition toward the metal foil laminated on one surface of theinsulating resin substrate to pass through the metal foil so as to forma first opening position having the diameter decreasing inward from theone surface of the insulating resin substrate and extending from the onesurface to near the center part of the insulating resin substrate.Alternatively, after forming a hole in which the diameter is nearly thesame as the diameter of the through-hole on the surface by etching inadvance at a predefined position of one side of the copper foil surfacelaminated on the insulating resin substrate (mask for laser), CO₂ gaslaser irradiation is performed using the hole as an irradiation mark toform a first opening portion in which the diameter decreases inward theinsulating resin substrate and that extends from the one surface of theinsulating resin substrate to near the center part.

Next, on a position out of alignment or off set by a predetermineddistance on the other surface of the insulating resin substrate facingthe predetermined position, laser irradiation is performed toward themetal foil laminated on the other surface of the insulating resinsubstrate, so as to form a second opening position in which the diameterdecreases inward from the other surface of the insulating resinsubstrate and that extends from the other surface to near the centerpart of the insulating resin substrate. Alternatively, after forming ahole whose diameter is almost same as the diameter of the through-holeon the surface by etching in advance at a predefined position of theother side of the copper foil surface laminated on the insulating resinsubstrate (a mask for laser), CO₂ gas laser irradiation is performedusing the hole as an irradiation mark to form a second opening portionin which the diameter decreases inward the insulating resin substrateand that extends from the other surface of the insulating resinsubstrate to near the center part.

When forming this second opening portion, the distance between thecenters of gravity of the first opening portion and the second openingportion (misalignment distance) is adjusted in accordance with thepositions of laser irradiation so that the first opening portion and thesecond opening portion is connected around the center part of theinsulating resin substrate to form a penetrating-hole for forming athrough-hole and that the plane area enclosed by a line connecting theneck portion is not parallel to the surfaces of the substrate (refer toFIG. 2).

The laser processing described above is performed with apulse-oscillating type carbonic acid gas laser device, and theprocessing conditions may be determined by the shape of apenetrating-hole for forming a through-hole in which the diameterdecreases inward from the surface of the insulating resin substrate, forexample, the pulse interval of 10-20 μs and the shots of 1-5 times.

With such laser processing conditions, the diameter of openings (theopening diameter of the first opening portion and the second openingportion) of a penetrating-hole for forming a through-hole may be set tobe in a range of 75-300 μm and the shortest diameter of the neck portionmay be set to be in a range of 50-250 μm, and the out-of-alignmentdistance of the centers of gravity for a penetrating-hole for forming athrough-hole may be set in a range of 5-30 μm.

(3) The desmere process is performed to remove resin residues left onthe side wall of the penetrating-hole formed in the step (2) describedabove.

This desmear process may be performed by wet processing such as chemicalreaction of acids or oxidants (e.g., chromic acid, hydrogenpermanganate), by dry processing such as oxygen plasma dischargetreatment, corona discharge treatment, ultra violet laser processing orby excimer laser processing.

(4) Then, nonelectrolytic plating process is performed to form anelectroless plated film on the inner wall of the penetrating-hole for athrough-hole and on the copper foil. In this case, the electrolessplated film may be made by using metals such as copper, nickel orsilver.

(5) In addition, using the electroless plated film formed in the (4)above as a lead, electrolytic plating process may be performed to forman electrolytic plated film on the electroless plated film covering thecopper foil of the substrate and also form a “drum shaped” (a shape oftwo connecting truncated cones) through-hole by gradually thickening aplated layer formed on the inner wall of the penetrating-hole and byfilling the electrolytic plated film inside the penetrating-hole.

(6) Then, an etching resist layer is formed on the electrolytic copperplated film formed on the substrate in the step (5) above. The etchingresist layer may be formed by coating a resist solution or laminating afilm prepared in advance. On this resist layer, a mask with a circuitdrawn in advance is placed, and exposed and developed to form an etchingresist layer. A conductor circuit pattern including a through-hole landis formed by etching parts of the metal layer where the etching resistis not formed.

As an etching solution, at least one kind of aqueous solution may beselected from aqueous solution of sulfuric acid hydrogen peroxide,peroxosulfates, copper chloride, and iron chloride.

As a pre-process to form a conductor circuit by the etching processdescribed above, the thickness may be adjusted by etching the entiresurface of the electrolytic copper plated film in advance, therebymaking the formation of fine patterns easier.

Using a printed wiring board according to an embodiment of the presentinvention prepared by following the steps (1) to (6) described above asa core substrate, a multilayer printed wiring board may be formed byforming a build-up wiring layer made of alternately laminatinginsulating resin layers and conductor circuit layers on one side or bothsides of the core substrate.

In such a multi-layer printed wiring board, a solder resist layer may beformed on the surface of the insulating resin layer on which theoutermost conductor circuit is formed, i.e., the outermost layer of thebuild-up wiring layer. In this case, a solder resist composition iscoated on the entire surface of the outermost layer of the printedwiring board, and after the coated film is dried, a photo mask film onwhich the opening portions of connection pads are drawn is placed onthis coated film and then exposed and developed to expose the connectionpad parts. In this case, a solder resist layer made of a dried film maybe laminated and then exposed and developed or by irradiated with laserto form openings.

On the connection pads exposed from the solder resist, acorrosion-resistant layer such as nickel-gold may be formed. Thethickness of the nickel layer may be preferably 1-7 μm and the thicknessof the gold layer may be preferably 0.01-0.1 μm. Other than thesemetals, nickel-paradium-gold, gold (single layer) or silver (singlelayer) may be used.

Then, on the connection pads, solder bodies are provided and withmelting and solidifying of the solder bodies, solder bumps are formedfor a multi-layer circuit substrate.

In the multi-layer printed wiring board formed in the steps describedabove, parts of the outermost conductor layer is formed as connectionpads in a predetermined pitch, and by setting the pitch of theseconnection pads similar to the pitch between adjacent through-holesfilled with plating in the core substrate, the wiring resistant viachips to be mounted on PKG may be reduced, providing an advantage ofensuring power supply.

EXAMPLES Example 1-1

(1) First, a circuit board (core) which serves as a part of amulti-layer printed wiring board is prepared. This circuit board is thesubstrate to be the center of multiple insulating layers to belaminated, and a double-sided copper clad laminate 10, which is obtainedby impregnating glass cloth with epoxy resin to form a prepreg made as Bstage and by heat-pressing copper foils thereto (refer to FIG. 4 (a)).

The thickness of the insulating resin substrate 12 is 300 μm and thethickness of the copper foils 14 is 3 μm. A copper foil which is thickerthan 3 μm may be used for this laminate and the thickness of the copperfoils may be adjusted to 3 μm by etching process.

(2) CO₂ gas laser irradiation is performed on a predetermined positionon the one surface of double-side circuit board 10 to form a firstopening portion 16 which passes through one copper foil 14 and extendsfrom the center of the insulating resin substrate 12 to a portion closeto the other surface (refer to FIG. 4 (b)), and also CO₂ gas laserirradiation is performed on the position which is out of alignment by 15μm from the position of the predetermined position on the other surfaceof double-side circuit board 10 to form a second opening portion 18which passes through the other copper foil 14 and extends from thecenter of the insulating resin substrate 12 to a portion close to theopposite surface so as to connect to the first opening portion 16. As aresult, by connecting the first opening portion 16 and the secondopening portion 18, a penetrating-hole 20 for forming a through-hole isformed.

In this embodiment, to form the penetrating-hole 20 for forming athrough-hole, for example, a high peak short pulse oscillating typecarbonic acid gas laser processing device (made by Hitachi Via Co., Ltd)may be used under the laser processing conditions in which the pulseinterval is 10-20 μs and the number of shots is 1-5. Thepenetrating-hole 20 is formed with 150 μm pitch, the opening diameter ofthe first opening portion 16 and the second opening portion 18 is almost150 μm the diameter of the neck portion around the center of thesubstrate, i.e., the minimum distance where the diameter is decreased atmost (indicated by “X” in FIG. 1) is almost 87 μm, and theout-of-alignment distance between the axes of the centers of gravity ofthe first opening portion 16 and the second opening portion 18 is 15 μm.

In the penetrating-hole 20 formed under such conditions, the firstopening portion 16 and the second opening portion 18, whose center axesare out of alignment with each other, are formed by parts of truncatedcones in which the inner walls are making a taper (inner angle) againstthe surfaces of the insulating resin substrate 12, and connected by thecommon connecting cross-section around the center of the substrate.

This connecting cross-section (refer to FIG. 2) is not parallel to thesurfaces of the insulating resin substrate. In addition, the centers ofgravity of adjacent connection cross-sections are preferably out ofalignment in the cross-section direction of the insulating resinsubstrate (refer to FIG. 3).

(3) Then, the desmere process is performed inside the penetrating-hole20 formed by laser processing to remove resin or particle residue lefton the inner wall with a physical methods such as O₂ plasma or CF₄plasma. In addition, soft etching process may be performed after washingthe desmere-processed substrate with water and subjecting it to aciddegreasing.

(4) Next, the desmere-processed substrate is immersed in the electrolesscopper plating aqueous solution prepared as described below to form theelectroless copper plated film 22 having a thickness of 0.6 μm on theentire surface of the copper foil 14 laminated on both sides of thesubstrate and the inner wall of the penetrating-hole 20 (Refer to FIG. 4(d)).

Electroless copper plating solution Copper sulfate: 0.03 mol/l EDTA:0.200 mol/l HCHO: 0.18 g/l NaOH: 0.100 mol/l α, α′-bipiridyl: 100 mg/lPolyethleneglycol: 0.10 g/l Solution temperature: 30-50° C. Duration:40-60 minutes

(5) Next, after the substrate is washed in 50° C. water for degreasingand washed in 25° C. water, and then washed again with sulfuric acid,electrolysis plating is performed under the condition below to form theelectrolysis plated film 24 (refer to FIG. 5 (a)).

Electrolysis copper plating solution Sulfuric acid: 2.24 mol/l Coppersulfate: 0.26 mol/l Additive agent: 19.5 ml/l Leveling agent: 50 mg/lGloss agent: 50 mg/l Electrolysis plating condition Current density: 1.0A/dm² Duration: 30-90 minutes Temperature: 22 ± 2° C.

(6) In FIG. 5, the electroless plated film 22 is not shown forsimplification. On the substrate on which the electrolytic copper platedfilm is formed, a film resist layer is attached. And on this resistlayer, a mask on which a circuit or circuits are drawn in advance ispositioned, and exposed and developed to form an etching resist layer 28(refer to FIG. 5 (b)). Then by etching parts of the metal layer wherethe etching resists is not formed, an inner layer conductor circuit 30having a thickness of 20-30 μm is formed on the front and back surfaceof the insulating resin substrate. Also, a through-hole land 32positioning on top of the through-hole 26 is formed. Thus, a coresubstrate is formed (refer to FIG. 5 (c)).

The wiring is formed with 100 through-holes connected via the conductorcircuit 30 and through-hole lands 32.

Then, an interlayer resin insulating layer or layers and a conductorlayer or layers are stacked alternately on the core substrate to formbuild-up wiring layers, thereby forming a multi-layered printed wiringboard.

(7) After washing with water, acid degreasing, and then soft-etching thesubstrate above, an etching solution is applied on both sides of thesubstrate with a spray and the surface of the inner layer conductorcircuit 30 (including the through-hole land 32) is etched to form arough surface (not shown) on the entire surface of the inner layerconductor circuit 30 (including the through-hole land 32).

The etching solution (made by MEC, MEC Etch Bond) containing of 10 partsby weight of imidazole copper (II) complex, 7 parts by weight of glycolacid and 5 parts by weight of potassium chloride is used as the etchingsolution.

(8) On both sides of the substrate, a resin film for interlayer resininsulating (e.g., ABF made by Ajinomoto) which is slightly bigger thanthe substrate is positioned, preliminary pressure-bonded under thecondition which is 0.4 MPa of pressure, 80° C. of temperature and 10seconds of pressure bonding time, and cut, and then, laminated by usinga vacuum laminating device to form an interlayer resin insulating layer36 as follows.

The resin film for an interlayer resin insulating layer is permanentlypress-bonded on the substrate under the condition of 67 Pa of vacuum,0.4 MPa of pressure, 80° C. of temperature, and 60 seconds of pressurebonding time and then thermally cured for 30 minutes at 170° C.

(9) Next, on the interlayer resin insulating layer 36, via a mask inwhich 1.2 mm thick of perforations are formed, with CO₂ gas laser of10.4 μm wavelength, under the conditions of 4.0 mm of beam diameter, tophat mode, 8.0 μsec of pulse interval, 1.0 mm diameter of a maskperforation hole, and 1-3 shots, openings 38 for via holes with 60 μmdiameter is formed in the interlayer resin insulating layer (refer toFIG. 5 (d)).

(10) The substrate in which the openings 38 for via holes are performedis immersed in an 80° C. solution containing 60 g/l of hydrogenpermanganate for 10 minutes to remove particles existing on the surfaceof the interlayer resin insulating layer 36, thus roughening the surfaceof the interlayer resin insulating layer 36 including the inner walls ofthe openings 38 for via holes (not shown).

(11) Then the substrate from the process above is immersed in aneutralization solution (made by Shipley) and washed in water.

In addition, on the surface of the substrate subjected the rougheningprocess (rough depth of 3 μm), by applying palladium catalyst, catalystnucleus are attached on the surface of the interlayer resin insulatinglayer 36 and the inner walls of the openings 38 for via holes (notshown). That is, the substrate above is immersed in the catalystsolution containing palladium chloride (PdCl₂) and stannous chloride(SnCl₂) and the catalyst is provided by separating out palladium metal.

(12) Next, the substrate with the catalyst is immersed in an electrolesscopper plating aqueous solution with the composition below to form anelectroless copper plated film having a thickness of 0.6-3.0 μm on theentire rough surface, thus obtaining the substrate in which theelectroless copper plated film (not shown) is formed on the surface ofthe interlayer resin insulating layer 36 including the inner walls ofthe openings 38 for via holes.

Electrolysis copper plating aqueous solution Copper sulfate: 0.03 mol/lEDTA: 0.200 mol/l HCHO: 0.18 g/l NaOH: 0.100 mol/l α, α′-bipiridyl: 100mg/l Polyethleneglycol: 0.10 g/lPlating ConditionSolution temperature: 30-50° C.Duration: 40-60 minutes

(13) A commercially available photosensitive dry film is laminated onthe substrate with the electroless copper plated film and a mask isplaced and exposed at 100 mJ/cm² and developed with 0.8% sodiumcarbonate aqueous solution to form a plated resists with a thickness of20 μm (not shown).

(14) Next, after the substrate is washed with 50° C. water fordegreasing, washed in 25° C. water, and then washed again with sulfuricacid, electrolysis plating is performed under the condition below toform an electrolysis plated film.

Electrolysis plating solution Sulfuric acid: 2.24 mol/l Copper sulfate:0.26 mol/l Additive agent: 19.5 ml/l Leveling agent: 50 mg/l Glossagent: 50 mg/l Electrolysis plating condition Current density: 1 A/dm²Duration: 65 minutes Temperature: 22 ± 2° C.

In this plating process, an electrolytic copper plating film having athickness of 20 μm is formed in the parts where plated resist is notformed, and also an electrolytic plated film is filled in the openings38 for via holes.

(15) In addition, after the plated resist is removed with 5% KOH, theelectroless plated film under the resist is dissolved and removed by theetching process with a mixed solution of sulfuric acid and hydrogenperoxide, thus forming filled vias 40 including via lands and anindependent external layer conductor circuit 44 (refer to FIG. 6 (a)).

(16) Then, a process same as (8) described above is performed to form arough surface (not shown) on the surfaces of the filled vias 40 and thesurface of the external layer conductor circuit 44.

(17) By repeating the steps from (8) to (15) described above, aninterlayer insulating layer 46, a conductor circuit 48 and filled vias50 are formed on further outside to obtain a multilayer wiring board(refer to FIG. 6 (b)).

(18) Next, on both sides of the multilayer wiring board, a commerciallyavailable solder resist composition is coated for 20 μm thickness, anddried for 20 minutes at 70° C. and for 30 minutes at 70° C., a photomask with 5 mm thick on which a solder resist opening pattern is drawnis closely attached on the solder resist layer, and the solder resistlayer exposed to 1000 mJ/cm² of ultra violet beam, and then developed toform openings 54 whose diameter is 60 μm (refer to FIG. 6 (c)).

Then, in addition, the solder resist layer is cured by subjecting toheating processes of one hour at 80° C., one hour for 100° C., one hourfor 120° C., and three hours for 150° C. to form a solder resist patternlayer 52 with 20 μm thick and openings.

(19) Then, the substrate on which the solder resist layer 52 is formedis immersed in nonelectrolytic nickel plating solution containing nickelchloride (2.3×10⁻¹ mol/l), sodium hypophosphite (2.8×10⁻¹ mol/l), sodiumcitrate (1.6×10⁻¹ mol/l) at pH=4.5 for 20 minutes to form a nickelplated layer with 5 μm thick (not shown) at opening portions 54. Inaddition, the substrate is immersed in a nonelectrolytic platingsolution containing potassium gold cynanide (7.6×10⁻³ mol/l), ammoniumchloride (1.9×10⁻¹ mol/l), sodium citrate (1.2×10⁻¹ mol/l), and sodiumhypophosphite (1.7×10⁻¹ mol/l) for 7.5 minutes at 80° C. to form a goldplated layer with 0.03 μm thick (not shown) on the nickel plated layer.

(20) In addition, on the opening 54 of the solder resist layer 52 on theside where an IC chip to be loaded, solder paste containing tin-lead isprinted and on the opening 54 of the solder resist layer 52 on the otherside, solder paste containing tin-antimony is printed, and then, byreflow at 230° C., the solder bumps 56 are formed, thus producing amultilayer printed wiring board (refer to FIG. 6 (d)).

Example 1-2

Other than forming a penetrating-hole 20 with the out-of-alignmentdistance between the gravity centers of the first opening portion 16 andthe second opening portion 18 to be 5 μm and the diameter of neckportion to be 76 μm by irradiating laser beam, a multiple layer printedwiring board is formed in a similar manner as Example 1-1.

Example 1-3

Other than forming a penetrating-hole 20 with the out-of-alignmentdistance between the gravity centers of the first opening portion 16 andthe second opening portion 18 to be 10 μm and the diameter of neckportion to be 80 μm by irradiating laser beam, a multiple layer printedwiring board is formed in a similar manner as Example 1-1.

Example 1-4

Other than forming a penetrating-hole 20 with the out-of-alignmentdistance between the gravity centers of the first opening portion 16 andthe second opening portion 18 to be 20 μm and the diameter of neckportion to be 91 μm by irradiating laser beam, a multiple layer printedwiring board is formed in a similar manner as Example 1-1.

Example 1-5

Other than forming a penetrating-hole 20 with the out-of-alignmentdistance between the gravity centers of the first opening portion 16 andthe second opening portion 18 to be 25 μm and the diameter of neckportion to be 97 μm by irradiating laser beam, a multiple layer printedwiring board is formed in a similar manner as Example 1-1.

Example 1-6

Other than forming a penetrating-hole 20 with the out-of-alignmentdistance between the gravity centers of the first opening portion 16 andthe second opening portion 18 to be 30 μm and the diameter of neckportion to be 110 μm by irradiating laser beam, a multiple layer printedwiring board is formed in a similar manner as Example 1-1.

Reference 1-1

Other than forming a penetrating-hole 20 with the out-of-alignmentdistance between the gravity centers of the first opening portion 16 andthe second opening portion 18 to be 3 μm and the diameter of neckportion to be 74 μm by irradiating laser beam, a multiple layer printedwiring board is formed in a similar manner as Example 1-1.

Reference 1-2

Other than forming a penetrating-hole 20 with the out-of-alignmentdistance between the gravity centers of the first opening portion 16 andthe second opening portion 18 to be 35 μm and the diameter of neckportion to be 95 μm by irradiating laser beam, a multiple layer printedwiring board is formed in a similar manner as Example 1-1.

Multilayer printed wiring boards of Examples 2-1 to 2-6 and References2-1 to 2-2 are formed in similar manners as Examples 1-1 to 1-6 andReferences 1-1 to 1-2, using double-sided copper clad laminates in whichthe insulating resin substrates are 100 μm thick and other than settingthe diameter of the first opening portion 16 and the second openingportion 18 as 75 μm, changing the opening diameter of the mask for laserand the conditions of the carbonic acid gas laser irradiation.

The out-of-alignment distances between axes of the gravity center andthe diameters of the neck portions in those examples and references arelisted in Table 1 and Table 2.

Also, multilayer printed wiring boards of Examples 3-1 to 3-6 andReferences 3-1 to 3-2 are formed in similar manners as Examples 1-1 to1-6 and References 1-1 to 1-2, using double-sided copper clad laminatesin which the insulating resin substrates are 500 μm thick and other thansetting the diameter of the first opening portion 16 and the secondopening portion 18 as 300 μm, changing the opening diameter of the maskfor laser and the conditions of the carbonic acid gas laser irradiation.

The out-of-alignment distances between axes of the gravity center andthe diameters of the neck portions in examples and references are listedin Table 2.

An evaluation test such as “A” described below is performed on themultilayer printed wiring boards produced according to Examples 1-1 to3-6 and References 1-1 to 3-2 described above. The result of thoseevaluation tests are shown in Table 1 and Table 2.

A. Heat Cycle Test

The connection resistance of wiring that connects 100 through-holesthrough the conductor circuits on the front and back of the coresubstrate of a multilayer printed wiring board is measured (initialvalue), and then, under the heat cycle condition where a single cycle of−55° C. for 5 minutes and 125° C. for 5 minutes, a cycle test isrepeated 1,000 times, and again the connection resistance is measured.

If the connection resistance variation (100×(the connection resistancevalue after the heat cycle−the initial value of connectionresistance)/the initial value of connection resistance) is 10% or less,it is passed (indicated by “O”) and if the connection resistancevariation is beyond 10%, it is failed (indicated by “X”).

As a result of the measurements, all examples passed while all referenceexamples failed.

For the core substrate of the multilayer printed wiring board in eachreference example, the presence of any voids in plating filled in thepenetrating-holes (through-holes) is observed using X-ray TV system(Shimadzu, product name “SMX-100”). 100 through-holes are selectedrandomly for the observation.

Many voids are confirmed in the core substrate in each referenceexample. It is believed that voids are formed when the out-of-alignmentdistance is small because the plating solution entering into thethrough-holes from the front and back of the core substrate is collidedhead-on. On the other hand, it is believed when the out-of-alignmentdistance is beyond 30 μm, the penetrating-holes are prone to deform asshown in FIGS. 7 (a)-(b) because the out-of-alignment distance is toobig.

TABLE 1 First and Misalignment second of axis of Neck Heat Board openingthe center of portion cycle Thickness diameter alignment diameter test(μm) (μm) (μm) (μm) result Example 300 150 15 87 ◯ 1-1 Example 300 150 576 ◯ 1-2 Example 300 150 10 80 ◯ 1-3 Example 300 150 20 91 ◯ 1-4 Example300 150 25 97 ◯ 1-5 Example 300 150 30 110 ◯ 1-6 Reference 300 150 3 74X 1-1 Reference 300 150 35 95 X 1-2 Example 100 75 15 37 ◯ 2-1 Example100 75 5 24 ◯ 2-2 Example 100 75 10 30 ◯ 2-3 Example 100 75 20 40 ◯ 2-4Example 100 75 25 45 ◯ 2-5 Example 100 75 30 49 ◯ 2-6

TABLE 2 First and Misalignment second of axis of Neck Heat Board openingthe center of portion cycle Thickness diameter alignment diameter test(μm) (μm) (μm) (μm) result Reference 100 75 3 24 X 2-1 Reference 100 7535 40 X 2-2 Example 500 300 15 162 ◯ 3-1 Example 500 300 5 156 ◯ 3-2Example 500 300 10 160 ◯ 3-3 Example 500 300 20 161 ◯ 3-4 Example 500300 25 168 ◯ 3-5 Example 500 300 30 170 ◯ 3-6 Reference 500 300 3 150 X3-1 Reference 500 300 35 160 X 3-2

From the result of the evaluation test A above, in the printed wiringboard produced according to each example, it is confirmed that crackingis prevented around the center part in which the diameter platedthrough-hole decreases and that good electric connectivity andmechanical strength are obtained.

In each example described above, the penetrating-holes formed in theinsulating resin substrate (core substrate) are filled with plating;however, the heat cycle test result is same for the amounts of theout-of-alignment even for through-holes (refer to FIG. 8) formed byforming through-hole conductors on inside walls of the penetrating-holesand then filling a filler in a gap enclosed by the through-holeconductors.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

1. A printed wiring board comprising: an insulating resin substratehaving a first surface and a second surface, the insulating resinsubstrate having at least one penetrating-hole passing through theinsulating resin substrate from the first surface to the second surface;a first conductor formed on the first surface of the insulating resinsubstrate; a second conductor formed on the second surface of theinsulating resin substrate; and a through-hole conductor structureformed in the penetrating-hole of the insulating resin substrate andelectrically connecting the first conductor and the second conductor,wherein the penetrating-hole has a first portion having an opening onthe first surface and a second portion having an opening on the secondsurface, and the first portion and the second portion are connected suchthat the first portion and the second portion are off-set from eachother, wherein the first portion has an axis of the center of gravity,the second portion has an axis of the center of gravity, the axis of thecenter of gravity of the first portion is out of alignment with the axisof the center of gravity of the second portion.
 2. The printed wiringboard according to claim 1, wherein the axis of the center of gravity ofthe first portion is out of alignment with the axis of the center ofgravity of the second portion by a distance in a range of 5 μm to 30 μm.3. The printed wiring board according to claim 1, wherein the insulatingresin substrate has a thickness which is in a range of 100 μm to 500 μm.4. The printed wiring board according to claim 1, wherein thepenetrating-hole has a neck portion connecting the first portion andsecond portion of the penetrating-hole, and the first portion and secondportion of the penetrating-hole taper from the openings on the firstsurface and second surface of the insulating resin substrate toward theneck portion.
 5. The printed wiring board according to claim 4, whereinthe first portion and second portion of the penetrating-hole have adiameter on the first surface and second surface of the insulating resinsubstrate in a range of 75 μm to 300 μm, respectively, and the neckportion of the penetrating-hole has a diameter in arrange of 50 μm to250 μm.
 6. The printed wiring board according to claim 1, wherein the atleast one penetrating-hole comprises a plurality of penetrating-holes,and the plurality of penetrating-holes are formed at a pitch in a rangeof 100 μm to 400 μm.
 7. The printed wiring board according to claim 1,wherein the through-hole conductor structure comprises a metal platingfilling the penetrating-hole.
 8. The printed wiring board according toclaim 1, wherein the through-hole conductor comprises an electrolessplated film or an electrolytic plated layer, or both an electrolessplated film and an electrolytic plated layer.
 9. The printed wiringboard according to claim 8, wherein: the through-hole conductorcomprises an electroless plated film on inner walls of the first portionand second portion of the penetrating-hole, and an electrolytic platedlayer over the electroless plated film.
 10. A printed wiring boardcomprising: a core substrate comprising an insulating resin substratehaving a first surface and a second surface, the insulating resinsubstrate having at least one penetrating-hole passing through theinsulating resin substrate from the first surface to the second surface,the core substrate further comprising a first inner conductor formed onthe first surface of the insulating resin substrate, a second innerconductor formed on the second surface of the insulating resin substrateand a through-hole conductor structure formed in the penetrating-hole ofthe insulating resin substrate and electrically connecting the firstconductor and the second conductor; a resin insulating layer formed overone of the first surface and second surface of the insulating resinsubstrate of the core substrate; and an external conductor circuitformed over the resin insulating layer and the core substrate, whereinthe penetrating-hole has a first portion having an opening on the firstsurface and a second portion having an opening on the second surface,and the first portion and the second portion are connected such that thefirst portion and the second portion are off-set from each other,wherein the first portion has an axis of the center of gravity, thesecond portion has an axis of the center of gravity, the axis of thecenter of gravity of the first portion is out of alignment with the axisof the center of gravity of the second portion.
 11. The printed wiringboard according to claim 10, wherein the axis of the center of gravityof the first portion is out of alignment with the axis of the center ofgravity of the second portion by a distance in a range of 5 μm to 30 μm.12. The printed wiring board according to claim 10, wherein theinsulating resin substrate has a thickness which is in a range of 100 μmto 500 μm.
 13. The printed wiring board according to claim 10, whereinthe penetrating-hole has a neck portion connecting the first portion andsecond portion of the penetrating-hole, and the first portion and secondportion of the penetrating-hole taper from the openings on the firstsurface and second surface of the insulating resin substrate to the neckportion.
 14. The printed wiring board according to claim 13, wherein thefirst portion and second portion of the penetrating-hole have a diameteron the first surface and second surface of the insulating resinsubstrate in a range of 75 μm and 300 μm, respectively, and the neckportion of the insulating resin substrate has a diameter in arrange of50 μm to 250 μm.
 15. The printed wiring board according to claim 10,wherein the at least one penetrating-hole comprises a plurality ofpenetrating-holes, and the plurality of penetrating-holes are formed ata pitch in a range of 100 μm to 400 μm.
 16. The printed wiring boardaccording to claim 10, wherein the through-hole conductor structurecomprises a metal plating filling the penetrating-hole.
 17. The printedwiring board according to claim 10, wherein the through-hole conductorcomprises an electroless plated film or an electrolytic plated layer, orboth an electroless plated film and an electrolytic plated layer. 18.The printed wiring board according to claim 17, wherein: thethrough-hole conductor comprises an electroless plated film on innerwalls of the first portion and second portion of the penetrating-hole,and an electrolytic plated layer over the electroless plated film.
 19. Aprinted wiring board comprising: an insulating resin substrate having afirst surface and a second surface opposing the first surface; apenetrating-hole passing through the insulating resin substrate from thefirst surface to the second surface, the penetrating hole comprising: afirst opening portion provided in the first surface and extending in adepth direction of the insulating resin substrate, the first openingportion having an opening on the first surface, a second opening portionprovided in the second surface and extending in a depth direction of theinsulating resin substrate, the second opening portion having an openingon the second surface which is not aligned with the opening on the firstsurface, and a neck portion formed at an area where the first openingportion and the second opening portion coincide such that thepenetrating through-hole passes through the insulating resin substrate;a first conductor formed on the first surface of the insulating resinsubstrate; a second conductor formed on the second surface of theinsulating resin substrate; and a through-hole structure comprising aplating conductor and formed in the penetrating-hole of the insulatingresin substrate such that the first conductor is electrically connectedto the second conductor by the through-hole structure.
 20. The printedwiring board according to claim 19, wherein: the through-hole conductorcomprises an electroless plated film on inner walls of the first openingportion and second opening portion of the penetrating-hole, and anelectrolytic plated layer over the electroless plated film.